1. Technical Field of the Invention
The present invention is generally directed at aircraft simulation systems. More specifically, this invention relates to a new system and method for creating turbulence, including progressive gust penetration by the rotor, for use in real-time piloted helicopter simulations.
2. Description of the Prior Art
The modern helicopter is no longer a vehicle used for simple missions where only its hovering capability is required. The helicopter, and rotorcraft in general, are increasingly faced with complex missions which push the aircraft to its design limits. One emerging requirement which challenges helicopters is stabilized flight through moderate or severe atmospheric turbulence to accomplish high workload mission tasks.
Rotorcraft, both civilian and military, now compete with many other forms of transportation and have often shown great reliability and productivity in mission performance. These rotorcraft may be required to perform missions such as hovering over, and landing on moving platforms in high turbulence conditions. In light of the dynamic flight conditions which are encountered routinely, it is essential to have the capability to accurately predict rotorcraft performance in a turbulent atmosphere.
It is therefore clear that there is a great and still unsatisfied need for real-time rotorcraft atmospheric turbulence models in flight simulators. One of the most common grievances that many pilots have is that while they do feel turbulence acting on the helicopter in the flight simulator, the motion is not similar to real atmospheric turbulence.
One of the most severe limitations of the current rotorcraft atmospheric turbulence models is the lack of understanding of the fundamental physics underlying the rotorcraft response to atmospheric turbulence, which is different from that of the fixed wing aircraft. There is a fundamental difference between the ways in which rotary wing aircraft, such as helicopters, and fixed wing aircraft, such as airplanes, experience atmospheric turbulence. The difference is primarily due to blade rotational velocity. Whereas a wing element of an airplane has pure translational motion as the airplane cuts through a turbulence field, a blade element of a helicopter has translational as well as rotational motion. In an airplane both the turbulence and the response to turbulence are stationary random processes which require conventional body-fixed sampling, By comparison, the turbulence as well as the response to turbulence in a helicopter are cyclostationary and require blade-fixed sampling.
The rotational velocity introduces appreciable spatial distribution of turbulence velocities over the rotor disc. For conventional helicopters operating at attitudes of 1000 feet or below, the rotational velocity effects are not negligible. Research efforts have been made to provide methods with an analytical basis to predict and simulate turbulence and response statistics. One such simulation method is based on representing turbulence sample functions in terms of multi-variable sinusoids with random phases and provides second-order statistics of covariances and spectral densities.
Another turbulence simulation method is described in Costello, M. F., "A Theory for the Analysis of Rotorcraft Operating in Atmospheric Turbulence," Proceedings of the 46th Annual National Forum of the American Helicopter Society, Washington, D. C., May 1990, pp. 1003-1015. This article describes turbulence over a rotor blade that is approximated by a series of radial shape functions. With this expansion, a stochastic state space model is formed where the system dynamics matrix and the control matrix are periodic with a period equal to the rotor rotational speed and input to the plant is generated by independent white noise sources.
The following articles reflect some of attempted methods for improving turbulence simulation models, all of which are incorporated herein by reference:
Howlett, J. J., "UH-60A Black Hawk Engineering Simulation Program: Vol. I -Mathematical Model," NASA CR-1 66309, Dec. 1981.
Prasad, J. V. R., Riaz, J., Gaonkar, G. H., Yingyi, D., "Real Time Implementation Aspects of a Rotorcraft Turbulence Simulation Method," 49th Annual Forum of the American Helicopter Society, St. Louis, Mo., May 1993, p. 459.
Dahl, H. J. and Faulkner, A. J., "Helicopter Simulation in Atmospheric Turbulence," Vertica, Vol.3, 1979, pp. 65-78.
Judd, M. and Newman, S. J., "An Analysis of Helicopter Rotor Response due to Gust and Turbulence," Vertica, Vol. 1, 1977, pp. 179188.
Dang, Y., G. Gaonkar, Prasad, J., Zhang, H., "Parallel Computing of Helicopter Response to Turbulence Toward Real-Time Implementation," 50th Annual Forum of the American Helicopter Society, Washington, D.C., Vol. 2, May 1994, pp. 869-882.
However, the foregoing conventional methods have not proven to be totally satisfactory in real-time simulations since they either take excessive execution time or they do not yield realistic turbulence according to pilot opinion.